Aquaporin-4 and transient receptor potential vanilloid 4 balance in early postnatal neurodevelopment

In the adult brain, the water channel aquaporin-4 (AQP4) is expressed in astrocyte endfoot, in supramolecular assemblies, called "Orthogonal Arrays of Particles" (OAPs) together with the transient receptor potential vanilloid 4 (TRPV4), finely regulating the cell volume. The present study aimed at investigating the contribution of AQP4 and TRPV4 to CNS early postnatal development using WT and AQP4 KO brain and retina and neuronal stem cells (NSCs), as an in vitro model of astrocyte differentiation. Western blot analysis showed that, differently from AQP4 and the glial cell markers, TRPV4 was downregulated during CNS development and NSC differentiation. Blue native/SDS-PAGE revealed that AQP4 progressively organized into OAPs throughout the entire differentiation process. Fluorescence quenching assay indicated that the speed of cell volume changes was time-related to NSC differentiation and functional to their migratory ability. Calcium imaging showed that the amplitude of TRPV4 Ca2+ transient is lower, and the dynamics are changed during differentiation and suppressed in AQP4 KO NSCs. Overall, these findings suggest that early postnatal neurodevelopment is subjected to temporally modulated water and Ca2+ dynamics likely to be those sustaining the biochemical and physiological mechanisms responsible for astrocyte differentiation during brain and retinal development.

Thermodynamics and S-Palmitoylation Dependence of Interactions between Human Aquaporin-4 M1 Tetramers in Model Membranes

Aquaporin-4 (AQP4) is a water channel protein found primarily in the central nervous system (CNS) that helps to regulate water-ion homeostasis. AQP4 exists in two major isoforms: M1 and M23. While both isoforms have a homotetrameric quaternary structure and are functionally identical when transporting water, the M23 isoform forms large protein aggregates known as orthogonal arrays of particles (OAPs). In contrast, the M1 isoform creates a peripheral layer around the outside of these OAPs, suggesting a thermodynamically stable interaction between the two. Structurally, the M1 isoform has an N-terminal tail that is 22 amino acids longer than the M23 isoform and contains two solvent-accessible cysteines available for S-palmitoylation at cysteine-13 (Cys-13) and cysteine-17 (Cys-17) in the amino acid sequence. Earlier work suggests that the palmitoylation of these cysteines might aid in regulating AQP4 assemblies. This work discusses the thermodynamic driving forces for M1 protein-protein interactions and how the palmitoylation state of M1 affects them. Using temperature-dependent single-particle tracking, the standard state free energies, enthalpies, and entropies were measured for these interactions. Furthermore, we present a binding model based on measured thermodynamics and a structural modeling study. The results of this study demonstrate that the M1 isoform will associate with itself according to the following expressions: 2[AQP4-M1]4 ↔ [[AQP4-M1]4]2 when palmitoylated and 3[AQP4-M1]4 ↔ [AQP4-M1]4 + [[AQP4-M1]4]2 ↔ [[AQP4-M1]4]3 when depalmitoylated. This is primarily due to a conformational change induced by adding the palmitic acid groups at Cys-13 and Cys-17 in the N-terminal tails of the homotetramers. In addition, a statistical mechanical model was developed to estimate the Gibbs free energy, enthalpy, and entropy for forming dimers and trimers. These results were in good agreement with experimental values.

Cellular and Molecular Mechanisms Activated by a Left Ventricular Assist Device

Left ventricular assist devices (LVADs) represent the final treatment for patients with end-stage heart failure (HF) not eligible for transplantation. Although LVAD design has been further improved in the last decade, their use is associated with different complications. Specifically, inflammation, fibrosis, bleeding events, right ventricular failure, and aortic valve regurgitation may occur. In addition, reverse remodeling is associated with substantial cellular and molecular changes of the failing myocardium during LVAD support with positive effects on patients' health. All these processes also lead to the identification of biomarkers identifying LVAD patients as having an augmented risk of developing associated adverse events, thus highlighting the possibility of identifying new therapeutic targets. Additionally, it has been reported that LVAD complications could cause or exacerbate a state of malnutrition, suggesting that, with an adjustment in nutrition, the general health of these patients could be improved.

AQP4-independent TRPV4 modulation of plasma membrane water permeability

Despite of the major role of aquaporin (AQP) water channels in controlling transmembrane water fluxes, alternative ways for modulating water permeation have been proposed. In the Central Nervous System (CNS), Aquaporin-4 (AQP4) is reported to be functionally coupled with the calcium-channel Transient-Receptor Potential Vanilloid member-4 (TRPV4), which is controversially involved in cell volume regulation mechanisms and water transport dynamics. The present work aims to investigate the selective role of TRPV4 in regulating plasma membrane water permeability in an AQP4-independent way. Fluorescence-quenching water transport experiments in Aqp4-/- astrocytes revealed that cell swelling rate is significantly increased upon TRPV4 activation and in the absence of AQP4. The biophysical properties of TRPV4-dependent water transport were therefore assessed using the HEK-293 cell model. Calcein quenching experiments showed that chemical and thermal activation of TRPV4 overexpressed in HEK-293 cells leads to faster swelling kinetics. Stopped-flow light scattering water transport assay was used to measure the osmotic permeability coefficient (Pf, cm/s) and activation energy (Ea, kcal/mol) conferred by TRPV4. Results provided evidence that although the Pf measured upon TRPV4 activation is lower than the one obtained in AQP4-overexpressing cells (Pf of AQP4 = 0.01667 ± 0.0007; Pf of TRPV4 = 0.002261 ± 0.0004; Pf of TRPV4 + 4αPDD = 0.007985 ± 0.0006; Pf of WT = 0.002249 ± 0.0002), along with activation energy values (Ea of AQP4 = 0.86 ± 0.0006; Ea of TRPV4 + 4αPDD = 2.73 ± 1.9; Ea of WT = 8.532 ± 0.4), these parameters were compatible with a facilitated pathway for water movement rather than simple diffusion. The possibility to tune plasma membrane water permeability more finely through TRPV4 might represent a protective mechanism in cells constantly facing severe osmotic challenges to avoid the potential deleterious effects of the rapid cell swelling occurring via AQP channels.

The ST2/IL-33 Pathway in Adult and Paediatric Heart Disease and Transplantation

ST2 is a member of interleukin 1 receptor family with soluble sST2 and transmembrane ST2L isoforms. The ligand of ST2 is IL-33, which determines the activation of numerous intracytoplasmic mediators following the binding with ST2L and IL-1RAcP, leading to nuclear signal and cardiovascular effect. Differently, sST2 is released in the blood and works as a decoy receptor, binding IL-33 and blocking IL-33/ST2L interaction. sST2 is mainly involved in maintaining homeostasis and/or alterations of different tissues, as counterbalance/activation of IL-33/ST2L axis is typically involved in the development of fibrosis, tissue damage, inflammation and remodeling. sST2 has been described in different clinical reports as a fundamental prognostic marker in patients with cardiovascular disease, as well as marker for the treatment monitoring of patients with heart failure; however, further studies are needed to better elucidate its role. In this review we reported the current knowledge about its role in coronary artery disease, heart failure, heart transplantation, heart valve disease, pulmonary arterial hypertension, and cardiovascular interventions.

Decoding Natural Astrocyte Rhythms: Dynamic Actin Waves Result from Environmental Sensing by Primary Rodent Astrocytes

Astrocytes are key regulators of brain homeostasis, equilibrating ion, water, and neurotransmitter concentrations and maintaining essential conditions for proper cognitive function. Recently, it has been shown that the excitability of the actin cytoskeleton manifests in second-scale dynamic fluctuations and acts as a sensor of chemophysical environmental cues. However, it is not known whether the cytoskeleton is excitable in astrocytes and how the homeostatic function of astrocytes is linked to the dynamics of the cytoskeleton. Here it is shown that homeostatic regulation involves the excitable dynamics of actin in certain subcellular regions of astrocytes, especially near the cell boundary. The results further indicate that actin dynamics concentrate into "hotspot" regions that selectively respond to certain chemophysical stimuli, specifically the homeostatic challenges of ion or water concentration increases. Substrate topography makes the actin dynamics of astrocytes weaker. Super-resolution images demonstrate that surface topography is also associated with the predominant perpendicular alignment of actin filaments near the cell boundary, whereas flat substrates result in an actin cortex mainly parallel to the cell boundary. Additionally, coculture with neurons increases both the probability of actin dynamics and the strength of hotspots. The excitable systems character of actin thus makes astrocytes direct participants in neural cell network dynamics.

Cell Volume Regulation Mechanisms in Differentiated Astrocytes

BACKGROUND/AIMS

The ability of astrocytes to control extracellular volume homeostasis is critical for brain function and pathology. Uncovering the mechanisms of cell volume regulation by astrocytes will be important for identifying novel therapeutic targets for neurological conditions, such as those characterized by imbalances to hydro saline challenges (as in edema) or by altered cell volume regulation (as in glioma). One major challenge in studying the astroglial membrane channels involved in volume homeostasis in cell culture model systems is that the expression patterns of these membrane channels do not resemble those observed in vivo. In our previous study, we demonstrated that rat primary astrocytes grown on nanostructured interfaces based on hydrotalcite-like compounds (HTlc) in vitro are differentiated and display molecular and functional properties of in vivo astrocytes, such as the functional expression of inwardly rectifying K+ channel (Kir 4.1) and Aquaporin-4 (AQP4) at the astrocytic microdomain. Here, we take advantage of the properties of differentiated primary astrocytes in vitro to provide an insight into the mechanism underpinning astrocytic cell volume regulation and its correlation with the expression and function of AQP4, Transient Receptor Potential Vanilloid 4
(TRPV4), and Volume Regulated Anion Channel (VRAC).

METHODS

The calcein quenching method was used to study water transport and cell volume regulation. Calcium imaging and electrophysiology (patch-clamp) were used for functional analyses of calcium dynamics and chloride currents. Western blot and immunofluorescence were used to analyse the expression and localization of the channel proteins of interest.

RESULTS

We found that the increase in water permeability, previously observed in differentiated astrocytes, occurs simultaneously with more efficient regulatory volume increase and regulatory volume decrease. Accordingly, the magnitude of the hypotonic induced intracellular calcium response, typically mediated by TRPV4, as well as the hypotonic induced VRAC current, was almost twice as high in differentiated astrocytes. Interestingly, while we confirmed increased AQP4 expression in the membrane of differentiated astrocytes, the expression of the channels TRPV4 and Leucine-Rich Repeats-Containing 8-A (LRRC8-A) were comparable between differentiated and non-differentiated astrocytes.

CONCLUSION

The reported results indicate that AQP4 up-regulation observed in differentiated astrocytes might promote higher sensitivity of the cell to osmotic changes, resulting in increased magnitude of calcium signaling and faster kinetics of the RVD and RVI processes. The implications for cell physiology and the mechanisms underlying astrocytic interaction with nanostructured interfaces are discussed.

Orthogonal arrays of particle assembly are essential for normal aquaporin-4 expression level in the brain

Astrocyte endfeet are endowed with aquaporin-4 (AQP4)-based assemblies called orthogonal arrays of particles (OAPs) whose function is still unclear. To investigate the function of OAPs and of AQP4 tetramers, we have generated a novel "OAP-null" mouse model selectively lacking the OAP forming M23-AQP4 isoform. We demonstrated that AQP4 transcript levels were not reduced by using qPCR. Blue native (BN)/SDS-PAGE and Western blot performed on OAP-null brain and primary astrocyte cultures showed the complete depletion of AQP4 assemblies, the selective expression of M1-AQP4-based tetramers, and a substantial reduction in AQP4 total expression level. Fluorescence quenching and super-resolution microscopy experiments showed that AQP4 tetramers were functionally expressed in astrocyte plasma membrane and their dimensions were reduced compared to wild-type assemblies. Finally, as shown by light and electron microscopy, OAP depletion resulted in a massive reduction in AQP4 expression and a loss of perivascular AQP4 staining at astrocyte endfeet, with only sparse labeling throughout the brain areas analyzed. Our study relies on the unique property of AQP4 to form OAPs, using a novel OAP-null mouse model for the first time, to show that (a) AQP4 assembly is essential for normal AQP4 expression level in the brain and (b) most of AQP4 is organized into OAPs under physiological conditions. Therefore, AQP4 tetramers cannot be used by astrocytes as an alternative to OAPs without affecting AQP4 expression levels, which is important in the physiological and pathological conditions in which OAP aggregation/disaggregation dynamics have been implicated.

Phase I and pharmacokinetic study of LY309887: a specific inhibitor of purine biosynthesis

PURPOSE

In this phase I trial in humans the safety and pharmacology of LY309887 on a weekly schedule combined with daily oral 5-mg doses of folic acid were evaluated.

BACKGROUND

LY309887 is an inhibitor of folate-dependent enzymes involved in de novo purine biosynthesis and has a broad preclinical antitumor activity. In murine systems, combining this agent with exogenous folic acid results in an enhanced therapeutic index.

METHODS

This study was a single-institution, open-label, clinical trial of dose escalation with toxicity and pharmacokinetic parameters determined. The dose range studied was 0.5-4 mg/m2 per week x6 and then a modified schedule weekly x3 every 6 weeks.

RESULTS

Dose-limiting toxicities were of delayed onset and associated with hematologic, neurologic, and mucosal effects. Pharmacokinetic parameters revealed dose linearity for AUC and Cmax. Low circulating levels of drug persisted for over 200 h. Urinary excretion accounted for approximately 50% of the parent drug but was highly variable. The urinary excretion was near maximal within 24 h of dosing.

CONCLUSIONS

The modified dosing schedule allowed repetitive dosing in patients. Further evaluation of the 2 mg/m2 per week x3 every 6 weeks with daily oral folate supplement as a potential phase II dose may be warranted.

Phase I trial of the combination of daily estramustine phosphate and intermittent docetaxel in patients with metastatic hormone refractory prostate carcinoma

BACKGROUND

To apply our preclinical findings of cytotoxic synergy with the combination of estramustine phosphate (EP) and docetaxel as the basis of treatment of hormone refractory metastatic prostate cancer in man. To determine the optimal dosage and the toxicities of these two agents for future trials.

PATIENTS AND METHODS

Seventeen patients with hormone refractory metastatic prostate cancer who were ambulatory with performance status < or = 2, normal marrow, renal and hepatic function were entered. Prior exposure to EP or a taxane were exclusion factors. EP was given orally at a dose of 14 mg/kg of body weight daily with concurrent docetaxel administered every 21 days as an intravenous infusion over 1 hour with dexamethasone 8 mg. PO BID for five days. EP dosages were kept static; docetaxel dosages were explored in a minimum of three patients per level for dosages of 40, 60, 70, and 80 mg/m2. Patients were evaluated weekly. Prostate specific antigen (PSA) was measured every three weeks.

RESULTS

Five patients were entered at a docetaxel dose of 40 mg/m2, three at 60 mg/m2, six at 70 mg/m2, and three at 80 mg/m2. Only one patient had received prior chemotherapy. Grades 1 or 2 hypocalcemia and hypophosphatemia were seen at all dosage levels. Other grade 2 or less toxicities not related to dosage included alopecia, anorexia, stomatitis, diarrhea, and epigastric pain. Dose limiting toxicities (DLT) as grade 4 leukopenia and grade 4 fatigue were seen at 80 mg/m2. The phase II dose was defined at 70 mg/m2 with rapidly reversible leukopenia and minor liver function abnormalities. At this dosing level, dose intensity was 88% and 86% over consecutive cycles for docetaxel and EP, respectively. Two vascular events occurred at this dose level (70 mg/m2): one arterial and the other venous. PSA decreases greater than 50% from baseline were seen in 14 of 17 patients at all dosage levels. Four of the 17 patients demonstrated a complete biochemical response (PSA < or = 4 ng/ml). One patient had a partial response with measurable lung and liver lesions.

CONCLUSION

EP given continuously with every three-week docetaxel at a dose of 70 mg/m2 is tolerable with evidence of antitumor activity based upon significant declines in PSA in the majority of patients and improvement of lung metastasis in one patient. Larger phase II studies of this combination in a homogenous population are warranted.

Therapy of refractory/relapsed acute myeloid leukemia and blast crisis of chronic myeloid leukemia with the combination of cytosine arabinoside, tetrahydrouridine, and carboplatin

Eight patients, of whom four had acute myeloid leukemia (AML) and four had chronic myeloid leukemia (CML) blast crisis, were treated with a combination of cytosine arabinoside (ARA-C: 1,600 mg/m2 in three patients, 1,200 mg/m2 in five patients), tetrahydrouridine (THU: 2,800 mg/m2 in two patients, 2,646 mg/m2 in one patient, 2,100 mg/m2 in five patients), and carboplatin (900 mg/m2 in four patients, 720 mg/m2 in one patient, 450 mg/m2 in three patients). As a result of this treatment, five of the eight patients became aplastic. Two of the four patients with CML blast crisis reverted to the chronic phase and two of the four patients with acute nonlymphocytic leukemia (ANLL) attained a remission (one partial remission and one complete remission). The major toxicities included myelosuppression, unacceptable hepatotoxicity, and diarrhea. Pharmacokinetics studies revealed that the addition of carboplatin did not significantly change the disposition of ARA-C. ARA-C levels were not significantly changed in comparison with those obtained in a prior study of ARA-C with THU (ARA-C plasma levels at 3 h, 2630 +/- 1170 ng/ml).